1. Field of the Invention
The present invention relates to a hard magnetic garnet material used for an optical communication system, a Faraday rotator, a method of manufacturing a Faraday rotator and a method of manufacturing a Bismuth-substituted rare earth iron garnet single crystal. The present invention also relates to an optical device using a Faraday rotator and an optical communication system provided with an optical device.
2. Description of the Related Art
Optical communications are currently becoming widespread at an increasing speed in contrast to telecommunications with small transmission capacity. As will be explained later, reasons may be summarized as follows: Optical communication allows, large volume transmission at high-speed, is advantageous in long-distance transmission because it requires fewer relays and is free of influences from electromagnetic noise.
Light is same as a radio wave used for TV or radio broadcasting or wireless communications in the sense that it is an electromagnetic wave. However, frequency of an electromagnetic wave used in optical communications is approximately 200 THz, equivalent to approximately 20000 times that of satellite broadcasting (approximately 10 GHz). Having a high frequency means having a short wavelength and being able to transmit more signals at high speed all the more. By the way, a wavelength (central wavelength) used in optical communications is 1.31 xcexcm (1310 nm) and 1.55 xcexcm (1550 nm).
As is well known, an optical fiber used for optical communications has a double structure of glass pieces of different refractive indices. Since light passing through the central core is repeatedly reflected inside the core, signals are transmitted correctly even if the optical fiber is curved. Moreover, since highly transparent, high purity silica glass is used for the optical fiber, optical communication attenuates only by approximately 0.2 dB per km. Accordingly, optical communications allow transmission of approximately 100 km without amplifiers and allows the number of relays to be reduced compared to telecommunications.
EMI (electromagnetic interference) becomes an issue in telecommunications, while communications using optical fibers are free of noise by electromagnetic induction, which allows information transmission with extremely high quality.
A current optical communication system converts an electric signal to an optical signal using an LD (laser diode) in an optical transmitter, transmits this optical signal through optical fibers and converts the optical signal to an electric signal using a PD (photodiode) in an optical receiver. Thus, elements essential to an optical communication system are LD, PD, optical fibers and optical connectors. Except for a relatively low-speed, short-distance communication system, a high-speed, long-distance communication system also requires, in addition to these device, optical transmission devices such as light amplifier and optical distributor, optical parts applied to these devices such as optical isolator, optical coupler, optical splitter, optical switch, optical modulator, optical attenuator, etc.
What plays a particularly important role in a high-speed, long-distance transmission or a multi-branched optical communication system is an optical isolator. In a current optical communication system, optical isolators are used in LD modules of an optical transmitter and relays. The optical isolator is an optical part that plays a role in transmitting electromagnetic waves only in one direction and blocking electromagnetic waves which are reflected at some midpoint and return. The optical isolator applies a Faraday effect which is a kind of magneto-optical effects. The Faraday effect refers to a phenomenon of rotation of the polarization plane of light which has passed through materials exhibiting a Faraday effect, that is, a Faraday rotator using such as rare earth iron garnet single crystal. The characteristic that the polarization direction of light rotates such as the Faraday effect is called xe2x80x9coptical rotary powerxe2x80x9d. Unlike normal optical rotary power, in the case of the Faraday effect, even if the light propagation direction is reversed, the original condition is not restored, but the polarization direction further rotates. An element using the phenomenon that the polarization direction of light rotates due to the Faraday effect is called a xe2x80x9cFaraday rotatorxe2x80x9d.
The function of the optical isolator will be explained taking an LD module as an example.
An LD is built in an optical transmitter as an LD module integrated with an optical fiber. The optical isolator is placed between the LD and optical fiber and has the function of preventing reflected light from returning to the LD using the Faraday effect. Reflected returning light refers to light which is emitted from the LD, slightly reflected by components such as optical connectors and returned. Reflected returning light causes noise to the LD. The optical isolator that lets light pass in only one direction eliminates this noise and maintains communication quality.
In the case of the LD in the optical transmitter, the vibration direction (polarization direction) of light emitted from the LD is determined to be only one direction, and therefore a polarization-dependent type optical isolator of a simple structure is used. FIG. 6 shows a basic configuration of a conventional polarization-dependent type optical isolator 10. The optical isolator 10 is comprised of a Faraday rotator 11 constructed of a garnet single crystal, a cylindrical permanent magnet 12 that surrounds the Faraday rotator 11 and magnetizes the Faraday rotator 11 and polarizers 13 and 14 that are placed at the front and back surfaces of the Faraday rotator 11. These polarizers 13 and 14 are placed at a relative angle of 45xc2x0. With the optical isolator 10, the direction in which light propagates will be called a xe2x80x9cforward directionxe2x80x9d, while the direction in which light is reflected and returned will be called a xe2x80x9cbackward directionxe2x80x9d.
Then, the mechanism whereby the optical isolator 10 blocks passage of light in the backward direction will be explained. FIG. 7A shows how light in the forward direction passes through the optical isolator 10, while FIG. 7B shows how light in the backward direction is prevented from passing through the optical isolator 10.
As shown in FIG. 7A, linearly polarized light that has passed through the polarizer 13 in the forward direction is rotated 45xc2x0 by the Faraday rotator 11 and passes through the polarizer 14 placed at a relative angle of 45xc2x0. On the other hand, as shown in FIG. 7B, in the backward direction, linearly polarized light that has passed through the polarizer 14 is further rotated 45xc2x0 by the Faraday rotator 11, and therefore the light cannot pass through the polarizer 13.
The polarization-dependent optical isolator 10 used for the LD module has been explained above. On the other hand, a polarization-independent optical isolator is also available, such as an optical isolator used for a light amplifier. In the case of a light amplifier, light from an optical fiber enters directly into the optical isolator, and so it is not possible to identify the polarization direction. For this reason, a polarization-independent optical isolator has been developed. The basic configuration thereof is well known and therefore explanations thereof will be omitted here. When the present invention simply refers to an xe2x80x9coptical isolatorxe2x80x9d, it has a concept including both the polarization-dependent and polarization-independent types.
The Faraday rotator affects the performance of the optical isolator. Therefore, the properties of materials composing the Faraday rotator are important factors in attaining a high performance optical isolator. The important factors in selecting materials composing the Faraday rotator include having large Faraday rotation angle with the wavelength used (1.31 xcexcm, 1.55 xcexcm in the case of optical fiber) and having high-level transparency. As a material satisfying these conditions, YIG (yttrium iron garnet: Y3Fe5O12) was used initially, but it was insufficient in terms of mass production and miniaturization.
Then, it was discovered that when a rare earth site of a garnet single crystal was substituted by bismuth (Bi), the Faraday rotary moment was improved drastically, and since then this Bi-substituted rare earth iron garnet single crystal came into use for the Faraday rotator.
By the way, the conventional bismuth-substituted rare earth iron garnet single crystal shows a constant value of Faraday rotation angle in a magnetic field exceeding saturated magnetic field. On the other hand, in a magnetic field lower than the saturated magnetic field, the Faraday rotation angle is proportional to the magnitude of the magnetic field and the Faraday effect disappears when the external magnetic field is removed. Thus, as shown in FIG. 6, the conventional optical isolator 10 would be provided with the permanent magnet 12 to apply a magnetic field greater than saturated magnetic field to the Faraday rotator 11.
For the optical isolator 10, there is also a demand for miniaturization and cost reduction as in the case of other devices and components. However, the presence of this permanent magnet 12 can be said to prevent miniaturization and cost reduction of the optical isolator 10.
Since the Faraday effect disappears when an external magnetic field is removed from the conventional bismuth-substituted rare earth iron garnet single crystal, it can be said to be a soft magnetic material. Therefore, placement of the permanent magnet 12 is indispensable. However, even if hard magnetism is provided for the bismuth-substituted rare earth iron garnet single crystal, that is, if the Faraday rotation angle can be kept therein with removing the external magnetic field, it is possible to omit the placement of the permanent magnet 12. Omission of the permanent magnet 12 will lead to miniaturization and cost reduction of the optical isolator or various devices and components using Faraday effect. Therefore, the development of a hard magnetic bismuth-substituted rare earth iron garnet single crystal is underway.
For example, Japanese Patent Laid-Open No. 6-222311 discloses a bismuth-substituted rare earth iron garnet single crystal grown using an LPE (Liquid Phase Epitaxial) method, the chemical composition of the single crystal being indicated by GdxRyBi3xe2x88x92xxe2x88x92yFe5xe2x88x92z (AlGa)zO12 (where R is at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Er, Tm, Yb, Lu and Y; x, y and z are numbers such as 1.0xe2x89xa6xxe2x89xa62.5, 0xe2x89xa6yxe2x89xa61.9, 0.5xe2x89xa6zxe2x89xa62.0), which maintains the Faraday rotation effect when an external magnetic field is applied in the direction crossing the plane of the single crystal to magnetically saturate the single crystal and then the external magnetic field is removed. It has been proven that when an external magnetic field exceeding saturation magnetization is applied, this single crystal maintains the Faraday rotation angle even if the external magnetic field is removed. However, it is only a Gd1.8Bi1.2Fe4.0Al0.5Ga0.5O12 single crystal that is specifically disclosed as this bismuth-substituted rare earth iron garnet single crystal and no other single crystals are disclosed specifically in the Japanese Patent Laid-Open No. 6-222311.
Furthermore, Japanese Patent Laid-Open No. 9-185027 discloses a hard magnetic bismuth-substituted rare earth iron garnet single crystal having the following chemical composition. By the way, Japanese Patent Laid-Open No. 9-185027 points out that inclusion of Gd, Tb and Dy should be avoided.
Bi1Eu1Ho1Fe4Ga1O12 
Bio0.75Eu1.5Ho0.75Fe4.1Ga0.9O12 
Bi1Eu2Fe4Ga0.5Al0.5O12 
Furthermore, Japanese Patent Laid-Open No. 9-328398 discloses a Faraday rotator obtained by applying a magnetization process to a bismuth-substituted rare earth iron garnet single crystal grown using an LPE method, whose chemical composition is indicated by Tb3xe2x88x92xBixFe5xe2x88x92yxe2x88x92zGayAlzO12 (where 1.1xe2x89xa6xxe2x89xa61.5, 0.65xe2x89xa6y+zxe2x89xa61.2, zxe2x89xa6y). Furthermore, Japanese Patent Laid-Open No. 10-31112 discloses a Faraday rotator obtained by applying a magnetization process to a bismuth-substituted rare earth iron garnet single crystal grown using an LPE method, whose chemical composition is indicated by Tb3xe2x88x92xxe2x88x92yHoxBiyFe5xe2x88x92xxe2x88x92wGazAlwO12 (where 0.40xe2x89xa6xxe2x89xa60.70, 1.30xe2x89xa6yxe2x89xa61.55, 0.7xe2x89xa6z+wxe2x89xa61.2, 0xe2x89xa6w/zxe2x89xa60.3). The Faraday rotators described in Japanese Patent Laid-Open No. 9-328398 and Japanese Patent Laid-Open No. 10-31112 exhibit rectangular hysteresis.
Furthermore, Japanese Patent Laid-Open No. 2000-180791 discloses that an optical isolator having a Faraday rotator with a rectangular magnetic hysteresis is obtained by setting the ratio of a coercive force value to a residual magnetization value to 1.5 or more and thereby suppressing generation of a magnetic domain having reverse magnetization due to a temperature dispersion of magnetic properties and dispersion of magnetic domain wall energy. Japanese Patent Laid-Open No. 2000-180791 further discloses that a coercive force and rectangularity of magnetic hysteresis are improved by applying heat treatment within a range of 600 to 1100xc2x0 C. to a bismuth-substituted rare earth iron garnet single crystal grown using an LPE method.
As shown above, Laid-Open No. 6-222311, Japanese Patent Laid-Open No. 9-185027 and Japanese Patent Laid-Open No. 9-328398 propose hard magnetic bismuth-substituted rare earth iron garnet single crystals. However, the above described bismuth-substituted rare earth iron garnet single crystals are limited to basic investigation into whether hard magnetism is exhibited or not and have no mention about Faraday rotary moment, temperature property and wavelength property of a Faraday rotation angle or insertion loss required for a Faraday rotator.
On the other hand, the bismuth-substituted rare earth iron garnet single crystal described in Japanese Patent Laid-Open No. 2000-180791 has a coercive force of 100 Oe (oersted) or more, but much stronger coercive force is demanded. This is because the stronger the coercive force, the higher-level property of the Faraday rotator is obtained in the hard magnetic bismuth-substituted rare earth iron garnet single crystal which features the ability to maintain the Faraday rotation angle even if the external magnetic field is removed.
In practical application of the Faraday rotator, the aforementioned Faraday rotary moment, temperature property, wavelength property, insertion loss and coercive force constitute important properties. Therefore, it is an object of the present invention to provide a hard magnetic bismuth-substituted rare earth iron garnet material with excellent Faraday rotary moment, temperature property and wavelength property, and insertion loss. It is another object of the present invention to provide a technology for stably manufacturing a high performance Faraday rotator with excellent coercive force, etc.
It is a further object of the present invention to provide an optical device such as optical isolator provided with a high performance Faraday rotator.
It is a still further object of the present invention to provide an optical communication system provided with such an optical device.
As described above, Faraday rotary moment, temperature property (temperature dependency), wavelength property (wavelength dependency), and insertion loss are important properties for practical application of a Faraday rotator.
Here, the Faraday rotation angle is proportional to the thickness of a material composing a Faraday rotator. A rotation angle per unit thickness is called xe2x80x9cFaraday rotary momentxe2x80x9d. Since the rotation angle of the Faraday rotator used for the optical isolator is 45xc2x0, it is possible to reduce the thickness of the Faraday rotator as Faraday rotary moment increases, which is more advantageous for miniaturization.
The optical isolator, which is one of optical devices, is not always used at a constant temperature and requires an operation guarantee in a temperature range of, for example, xe2x88x9240xc2x0 C. to +85xc2x0 C. As the temperature dependency of the Faraday rotation angle becomes smaller, operation in a wider temperature range is possible. Therefore, the temperature dependency of the Faraday rotation angle, that is, temperature property is also an important property.
Wavelengths used for a current optical isolator are 1.31 xcexcm (1310 nm) and 1.55 xcexcm (1550 nm), but these are only central wavelengths. That is, there is a certain width in wavelengths of light actually emitted from an LD. Thus, the wavelength dependency of the Faraday rotation angle, that is, wavelength property is also a necessary property. Above all, it is an extremely important property when adopting a large volume transmission technology using wavelength multiplexing for optical communications.
Furthermore, attenuation of emitting light with respect to incident light is called xe2x80x9cinsertion lossxe2x80x9d. To secure high quality information transmission, the Faraday rotator is required to reduce insertion loss. Insertion loss of a Faraday rotator consists of light absorption loss of a material composing the Faraday rotator and reflection loss of an interface due to a difference in refractive index between the material and air. Reflection loss can be reduced to a negligible level by applying anti-reflective coating to the Faraday rotator surface. Thus, the insertion loss of the optical isolator is light absorption loss of the Faraday rotator. This light absorption loss is determined by light absorption of ions composing a bismuth-substituted rare earth iron garnet material.
The present inventors have examined how to realize hard magnetism with excellent Faraday rotary moment, temperature property, wavelength property and insertion loss. As a result, the present inventors have discovered that an unprecedentedly new chemical composition necessarily containing Gd, Tb and Yb at rare earth sites is effective for a bismuth-substituted rare earth iron garnet material. The present invention is based on this knowledge, provides a hard magnetic garnet material characterized by having a chemical composition of (Bi3xe2x88x92axe2x88x92bxe2x88x92cGdaTbbYbc) Fe(5xe2x88x92w)MwO12 (where, M is at least one element selected from the group consisting of Ga, Al, Ge, Sc, In, Si and Ti, 0.5xe2x89xa6a+b+cxe2x89xa62.5, 0.2xe2x89xa6wxe2x89xa62.5), and exhibiting rectangular magnetic hysteresis.
For the hard magnetic garnet material of the present invention, it is preferable to set 1.0xe2x89xa6a+b+cxe2x89xa62.3 and 0.3xe2x89xa6wxe2x89xa62.0, or further preferable to set 0.1xe2x89xa6axe2x89xa61.5, 0.3xe2x89xa6bxe2x89xa62.0, 0.1xe2x89xa6cxe2x89xa61.5 and 0.4xe2x89xa6wxe2x89xa61.5.
The hard magnetic garnet material of the present invention allows Faraday rotary moment in a temperature range of xe2x88x9240xc2x0 C. to +85xc2x0 C. with a wavelength of 1550 nm to be set to 700xc2x0/cm or more. Furthermore, the hard magnetic garnet material of the present invention can reduce insertion loss at room temperature with a wavelength of 1550 nm to 0.1 dB or less. The hard magnetic garnet material of the present invention can further substantially maintain the aforementioned Faraday rotary moment even after an external magnetic field equal to or greater than saturation magnetization of the hard magnetic garnet material is applied and then the external magnetic field is removed. The hard magnetic garnet material of the present invention can set the temperature property of the Faraday rotation angle in a temperature range of xe2x88x9240xc2x0 C. to +85xc2x0 C. with a wavelength of 1550 nm to 13% or less of its target value. The hard magnetic garnet material of the present invention can further set the wavelength property of the Faraday rotation angle at room temperature with a wavelength of 1500 to 1600 nm to 8% or less of its target value.
The present invention provides the following Faraday rotator to which the above described hard magnetic garnet material is applied. That is, the Faraday rotator of the present invention is a Faraday rotator that uses a bismuth-substituted rare earth iron garnet single crystal and rotates the polarization plane of incident light, characterized in that the aforementioned single crystal always contains Gd, Tb and Yb as rate earth elements, exhibits substantially rectangular magnetic hysteresis, has Faraday rotary moment of 700xc2x0/cm or more in a temperature range of xe2x88x9240xc2x0 C. to +85xc2x0 C. with a wavelength of 1550 nm and has insertion loss of 0.1 dB or less at room temperature with a wavelength of 1550 nm. It is also characterized in that the temperature property of the Faraday rotation angle in a temperature range of xe2x88x9240xc2x0 C. to +85xc2x0 C. with a wavelength of 1550 nm is 13% or less of its target value and the wavelength property of the Faraday rotation angle at room temperature with a wavelength of 1500 nm to 1600 nm is 8% or less of its target value.
The single crystal of the Faraday rotator of the present invention can contain at least one element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Dy, Lu, Tm, Er, Ho, Y, and Ca.
The Faraday rotator of the present invention can set the Faraday rotary moment in a temperature range of xe2x88x9240xc2x0 C. to +85xc2x0 C. with a wavelength of 1550 nm to 800xc2x0/cm or more, temperature property thereof in a temperature range of xe2x88x9240xc2x0 C. to +85xc2x0 C. with a wavelength of 1550 nm to 11% or less of its target value, wavelength property thereof at room temperature with a wavelength of 1550 nm to 1600 nm to 7% or less of its target value, and insertion loss at room temperature and a wavelength of 1550 nm to 0.07 dB or less.
The present invention also provides the following optical device using the above described Faraday rotator. The optical device of the present invention is basically composed of a first optical element into which forward light enters, a second optical element placed opposite to the first optical element in a predetermined distance from which forward light is emitted and a Faraday rotator placed between the first optical element and second optical element, which rotates the polarization plane of light that has passed through the first optical element and emits the light toward the second optical element and blocks passage of backward light that has passed through the second optical element. The optical devices referred to here include a wide range of devices such as optical isolator, optical circulator, optical attenuator, magneto-optical field sensor and optical switch, etc. Furthermore, polarization separators such as polarizer, rutile can be used as the first optical element and second optical element. The optical device of the present invention is characterized in that it is constructed of a bismuth-substituted rare earth iron garnet single crystal having a chemical composition of (Bi3xe2x88x92axe2x88x92bxe2x88x92cGdaTbbYbc) Fe(5xe2x88x92w)MwO12 (where, M is at least one element selected from the group consisting of Ga, Al, Ge, Sc, In, Si and Ti, 0.5xe2x89xa6a+b+cxe2x89xa62.5, 0.2xe2x89xa6wxe2x89xa62.5), and characterized in that this bismuth-substituted rare earth iron garnet single crystal exhibits rectangular magnetic hysteresis.
Optical devices such as optical isolators are used for optical transmitters in an optical communication system as described above. The present invention also proposes that the optical device of the present invention should be applied to this optical communication system. This proposal is an optical communication system provided with an optical transmitter that issues an optical signal converted from an electric signal, an optical transmission line that transmits the optical signal issued form the optical transmitter and an optical receiver that receives the optical signal sent through the optical transmission line and converts the received optical signal to an electric signal, characterized in that the optical transmitter includes an electro-optic converter that converts the electric signal to the optical signal and an optical device placed between the electro-optic converter and the optical transmission line, a Faraday rotator composing the optical device is constructed of a bismuth-substituted rare earth iron garnet single crystal having a chemical composition of (Bi3xe2x88x92axe2x88x92bxe2x88x92cGdaTbbYbc) Fe(5xe2x88x92w)MwO12 (where, M is at least one element selected from the group consisting of Ga, Al, Ge, Sc, In, Si and Ti, 0.5xe2x89xa6a+b+cxe2x89xa62.5, 0.2xe2x89xa6wxe2x89xa62.5) and this bismuth-substituted rare earth iron garnet single crystal exhibits rectangular magnetic hysteresis.
In the optical communication system, for example, a light amplifier may be placed on the optical transmission line made of an optical fiber. An optical device such as an optical isolator may also be used for this light amplifier. The optical device of the present invention may also be used for this optical device. That is, for the optical communication system of the present invention, a light amplifier is placed on the optical transmission line, the light amplifier includes an optical device that receives the optical signal transmitted on the optical transmission line and rotates the polarization plane of the received optical signal and amplification means for amplifying the optical signal that has passed through the optical device, the Faraday rotator making up the optical device can be constructed of a bismuth-substituted rare earth iron garnet single crystal having a chemical composition of (Bi3xe2x88x92axe2x88x92bxe2x88x92cGdaTbbYbc) Fe(5xe2x88x92w)MwO12 (where, M is at least one element selected from the group consisting of Ga, Al, Ge, Sc, In, Si and Ti, 0.5xe2x89xa6a+b+cxe2x89xa62.5, 0.2xe2x89xa6wxe2x89xa62.5). Moreover, this bismuth-substituted rare earth iron garnet single crystal preferably exhibits rectangular magnetic hysteresis.
Furthermore, as shown above, a coercive force is an important property for practical application of the Faraday rotator using a hard magnetic garnet material. The present inventors have made various analyses to obtain a bismuth-substituted rare earth iron garnet material with hard magnetism and an excellent coercive force. As a result, the present inventors have discovered that the bismuth-substituted rare earth iron garnet material exhibits ideal rectangular magnetic hysteresis and at the same time improved coercive force by applying heat treatment while applying an external magnetic field (hereinafter referred to as xe2x80x9cmagnetic heat treatmentxe2x80x9d as appropriate). That is, the present invention is a method of manufacturing a Faraday rotator using a bismuth-substituted rare earth iron garnet single crystal exhibiting substantially rectangular magnetic hysteresis and is characterized by including a single crystal growing step of growing a single crystal and a magnetic heat treatment step of applying heat treatment while applying an external magnetic field to the single crystal. Here, as the chemical composition of the single crystal to be grown in the single crystal growing step, adopting the chemical composition recommended by the present invention, that is, (Bi3xe2x88x92axe2x88x92bxe2x88x92cGdaTbbYbc) Fe(5xe2x88x92w)MwO12 (where, M is at least one element selected from the group consisting of Ga, Al, Ge, Sc, In, Si and Ti, 0.5xe2x89xa6a+b+cxe2x89xa62.5, 0.2xe2x89xa6wxe2x89xa62.5), makes it possible to obtain a Faraday rotator with a high coercive force with high performance also in aspects of Faraday rotary moment, temperature property, wavelength property and insertion loss. Furthermore, in the magnetic heat treatment step of the present invention, applying magnetic heat treatment at temperatures of 1100xc2x0 C. or lower is effective in improving the coercive force. Furthermore, it is desirable to cool down the single crystal while applying an external magnetic field after maintaining the temperature range in the magnetic heat treatment step. Furthermore, it is preferable to set the external magnetic field in the magnetic heat treatment step to 300 Oe or more and it is preferable to adopt an irreducible atmosphere as the heat treatment atmosphere in the magnetic heat treatment step.
Furthermore, the present invention provides a Faraday rotator characterized in that it uses a bismuth-substituted rare earth iron garnet single crystal and rotates the polarization plane of incident light, the single crystal having a chemical composition of (Bi3xe2x88x92axe2x88x92bxe2x88x92cGdaTbbYbc) Fe(5xe2x88x92w)MwO12 (where, M is at least one element selected from the group consisting of Ga, Al, Ge, Sc, In, Si and Ti, 0.5xe2x89xa6a+b+cxe2x89xa62.5, 0.2xe2x89xa6wxe2x89xa62.5), and characterized by having a coercive force of 600 Oe or more at room temperature through magnetic heat treatment. This is based on the knowledge of the present inventors that the magnetic properties improve by constructing the bismuth-substituted rare earth iron garnet single crystal using Gd, Tb and Yb as essential elements.
The present invention further provides a method of manufacturing a bismuth-substituted rare earth iron garnet single crystal exhibiting substantially rectangular magnetic hysteresis characterized by including a step of growing a single crystal using a liquid phase epitaxial growth method and a step of making said single crystal a single magnetic domain while heating or cooling this single crystal. In the step of making said single crystal a single magnetic domain, it is effective to heat or cool down the single crystal to the temperatures of 1100xc2x0 C. or lower while applying an external magnetic field of 300 Oe or more.
The present invention also provides the following optical device using a Faraday rotator. The optical device of the present invention uses, as basic components, a first optical element into which forward light enters, a second optical element placed opposite to the first optical element in a predetermined distance from which forward light is emitted and a Faraday rotator placed between the first optical element and second optical element, which rotates the polarization plane of light that has passed through the first optical element and emits the light toward the second optical element and blocks passage of backward light that has passed through the second optical element. For the optical device according to the present invention, it is possible to use a Faraday rotator constructed of a bismuth-substituted rare earth iron garnet single crystal, which is heated or cooled with an external magnetic field applied thereto in the direction in which the magnetic field is formed approximately parallel to forward light. This Faraday rotator exhibits rectangular magnetic hysteresis and has coercive force of 600 Oe or more at room temperature, and therefore the optical device according to the present invention, for example, optical isolator realizes high performance isolation.
On the other hand, the Faraday rotator is manufactured by polishing a bismuth-substituted rare earth iron garnet single crystal (hereinafter referred to as xe2x80x9cgarnet single crystalxe2x80x9d as appropriate or simply as xe2x80x9csingle crystalxe2x80x9d) formed using an LPE method to a predetermined thickness and then cutting it. The present inventors have conducted various analyses from standpoints other than the chemical composition to obtain a high performance Faraday rotator and have discovered that properties of the cross section of the garnet single crystal affect the performance of the Faraday rotator. Here, properties of the section of the garnet single crystal include the presence/absence of chipping, etc. Chipping refers to a phenomenon that edges of the section of the garnet single crystal become chipped when the garnet single crystal is cut. Thus, to obtain a high performance Faraday rotator, not only properties of the material composing the Faraday rotator is important but also how to reduce chipping when cutting the garnet single crystal constitutes an important key.
This chipping is especially an important issue for a hard magnetic garnet single crystal. This is because chipping that occurs when the magnetic garnet single crystal is cut becomes a kind of crystalline defect, which reduces a coercive force significantly. Here, as described above, the coercive force is an important element for the hard magnetic garnet single crystal. Furthermore, the coercive force and chipping when the magnetic garnet single crystal is cut have a close relationship and suppressing chipping when cutting the garnet single crystal is extremely important in improving the coercive force.
The present inventors have tried various cutting methods to suppress chipping when cutting the garnet single crystal and has come to discover that cutting using a wire saw is very effective. That is, the present invention provides a method of manufacturing a Faraday rotator which uses a bismuth-substituted rare earth iron garnet single crystal and rotates the polarization plane of incident light, characterized by including a single crystal growing step of growing a single crystal and a cutting step of cutting the single crystal obtained in this single crystal growing step, using a wire saw. The Faraday rotator manufacturing method according to the present invention is effective not only for a hard magnetic material having the aforementioned chemical composition recommended by the present invention but also for hard magnetic materials having other chemical compositions. Furthermore, the cutting method using a wire saw according to the present invention is not limited to hard magnetic materials but is also applicable to conventional soft magnetic materials. Especially when the Faraday rotator adhered to other optical elements by means of resin, etc. is cut using a wire saw, it is possible to effectively suppress chipping or detachment of optical elements during cutting.
Furthermore, the present invention also provides a Faraday rotator using a bismuth-substituted rare earth iron garnet single crystal, which includes front and back surfaces placed opposite to each other in a predetermined distance and sides formed around these front and back surfaces, characterized in that fine projections and depressions are formed uniformly on at least one side of the sides. To form uniform fine projections and depressions on the sides of the bismuth-substituted rare earth iron garnet single crystal, for example, a wire saw can be used. In addition, at least one side of the sides in the single crystal has an isotropic pattern. For the Faraday rotator according to the present invention, it is effective to use a single crystal exhibiting substantially rectangular magnetic hysteresis.
Furthermore, the present invention also provides the following optical device using a Faraday rotator. The optical device of the present invention uses, as basic components, a first optical element into which forward light enters, a second optical element placed opposite to the first optical element in a predetermined distance from which forward light is emitted and a Faraday rotator placed between the first optical element and second optical element, which rotates the polarization plane of light that has passed through the first optical element and emits the light toward the second optical element and blocks passage of backward light that has passed through the second optical element. For the optical device according to the present invention, the single crystal is sandwiched between the first optical element and the second optical element with the first optical element and the second optical element are adhered to the single crystal using an adhesive such as resin. Furthermore, since the Faraday rotator of the optical device according to the present invention exhibits rectangular magnetic hysteresis and has a coercive force of 350 Oe or more at room temperature, the optical device of the present invention exhibits high performance.